This invention relates to a hologram reproduction apparatus and method for reproducing multiple recorded data in a hologram recording medium in accordance with an angle multiple method, and more particularly to assurance of the compatibility in such a case that a hologram recording medium is re-set or exchanged with another hologram recording medium recorded by another apparatus.
Conventionally, upon recording of a hologram data storage, the hologram recording medium is disposed in a region in which two light fluxes of signal light and reference light interfere with each other to record interference fringes of the two light fluxes on the hologram recording medium. In order to assure a high recording density on the hologram recording medium upon such recording, various multiple methods such as the angle multiple method mentioned above, a shift multiple method, a speckle multiple method and a phase code multiple method are used for multiple recording of holograms.
In a hologram recording apparatus according to the angle multiple method, two light fluxes of signal light and reference light are irradiated upon a hologram recording medium. Further, every time the signal light is spatially modulated (intensity modulated) with a data page displayed on a spatial light modulator (SLM), the incidence angle (recording angle) of the reference light to the hologram recording medium is varied to multiple record different recording information in the same recording region.
In the hologram recording apparatus of the angle multiple method described, if the variation pitch of the recording angle of the reference light is changed at steps of 0.01 degree within a range of several degrees in the maximum (changed little by little, for example, within the range from 88 to 92 degrees) to multiple record different recording information (data pages) of the same recording area successively, then a plurality of different pieces of recording information such as 50-multile information can be recorded in the same recording area.
On the other hand, a hologram reproduction apparatus which is paired with such a hologram recording apparatus as described above is configured such that the angle of the surface of the hologram recording medium with respect to a light flux of reproduction reference light is varied little by little to reproduce the recording information multiple recorded in the same recording area. It is to be noted that the angle of a light flux of reproduction reference light in reproduction of a hologram recording medium recorded in accordance with such an angle recording method as described above with respect to the surface of the hologram recording medium is hereinafter referred to suitably as “reproduction angle”. Further, an angle which makes a reference to the reproduction angle such as, for example, a reproduction angle when it coincides with a normal to the surface of the hologram recording medium is hereinafter referred to suitably as “reference reproduction angle”. According to the hologram reproduction apparatus of the angle multiple method, recording information multiplex recorded in the same area can be reproduced for individual reproduction angles by varying the reproduction angle a little corresponding to that in the case of the recording angle from the reference reproduction angle.
In this manner, according to the hologram recording apparatus and the hologram recording method of the angle multiple type, a large number of pieces of recording information can be recorded for the individual recording angles of the reference right in the same recording area and besides the pieces of recording information multiple recorded in this manner can be reproduced separately from each other. Consequently, the recording density and the recording capacity can be increased remarkably.
Incidentally, according to hologram recording, information is usually recorded three-dimensionally in a recording area of a hologram recording medium but is recorded, on the surface of the hologram recording medium, in a two-dimensional area (recording face). Therefore, if the set position of the recording medium is displaced even a little between that upon recording and that upon reproduction, then it becomes less easy to find out the recording face. This is because, if the position of the hologram recording medium varies, then information of the wavefront changes between reference light upon recording and reference light upon reproduction and a reproduction image cannot be obtained thereby. On the other hand, although it is necessary to reduce the distance between adjacent recording areas if it is tried to raise the recording density in a hologram recording medium, as the recording areas come close to each other, then even if the hologram recording medium is displaced a little from a predetermined position upon reproduction, there is the possibility that the recording information of the different recording angle may be reproduced. Accordingly, when the hologram recording medium is to be reproduced, it is necessary to set the hologram recording medium at the same position as that upon recording, and it is demanded to perform the positioning precisely. If such precise positioning cannot be achieved, then when the hologram recording medium is re-set or is reproduced by another apparatus or when a hologram recording medium recorded by another apparatus is reproduced, the compatibility cannot be assured. This makes a factor of obstructing the placement of the angle multiple method into practical use.
Thus, the recording angle when data are recorded on a particular recording face on which data are recorded, for example, at a first recording angle of a hologram recording medium is set as a reference recording angle. Then, the recording angle after the data are recorded on the particular recording face is successively varied by a predetermined angle with reference to the set reference recording angle and fixed at the angle. Therefore, the reference recording angle set by mechanical conditions or set conditions of a recording angle variation section, an optical system and so forth of the hologram recording apparatus side and the reference recording angle on the hologram recording medium side are made coincide with each other upon particular recording such as upon first recording to enhance the compatibility. The technique for the enhancement of the compatibility just described is disclosed in International Publication Patent No. WO03/098362 (hereinafter referred to as Patent Document 1).